TiO2 Thin Film Deposition by RF Reactive Sputtering for n-i-p Planar Structured Perovskite Solar Cells

TiO2 thin film typically used as an electron transport layer (ETL) in perovskite solar cells has many advantages such as high refractive index, good photocatalytic properties, excellent chemical stability, and low cost. TiO2 thin films are typically formed by a solution process, but it is difficult to achieve large area coating, accurate thickness control, and mass production with this process. Here, we demonstrated that radiofrequency (RF)-sputtered TiO2 is an effective replacement for use as the ETL for the perovskite solar cells in order to overcome the disadvantages of the solution process. The TiO2 layer was deposited on the substrate by reactive sputtering (RS) with a high-purity Ti metal target. The RFRS TiO2 thin film was systematically characterized and compared with spin-coated TiO2 by means of various analytical techniques. The transmittance of 20-nm-thick as-deposited TiO2 on an indium tin oxide (ITO)-coated glass substrate was 75-80 % in the visible range. After annealing, the amorphous phase was converted into an anatase structure in RFRS TiO2. In the case of the spin-coated TiO2 layer, indium diffusion from the ITO layer and an increase in sheet resistance with the annealing temperature were observed. On the other hand, the RFRS TiO2 had a denser and more uniform surface, and it could be annealed at a lower temperature. Therefore, it was able to block indium diffusion and increase the VOC and FF values. Finally, the device efficiency of the perovskite solar cell was improved from 16.04 to 17.46 % when using RFRS TiO2 as the ETL.